Could you please tell me the detrimental effects if two diodes were in circuit rather than one?

I realize this, and yet it originally had one diode and no filtering what so ever. Ripple would have been relatively high with half wave rectification.
So what should I have labeled the output voltage. 13.4, 16V with ripple???
Regards,
Relayer

Edit:
@ evilkitty
The more I look at that thermal disconnect, it looks more like a mercury tilt switch.
Is one of the components pigtail attached to the top of the transformer?

I already explain that it is pulse charging circuit. That D2you have in the circuit is just a waste of power dissipation.

If we are going for vintage wouldn't original working order be better? as opposed to adding a pcb with a few components in it?
to my understanding it just needs to ~$0.50 diode to be vintage
now if wanted to upgrade this design i could slap a few caps in this thing and rewire it, but then it would not be original in its design, all of the components are on the ground side of the circuit

You have to remember that when people restore vintage radios and other appliances, its extremely rare that they can get original parts again. Therefore they have to use modern components to do their repairs.
Some actually hide the modern component inside the original by hollowing it out. Paper capacitors were a favorite. But inevitably they have to replace some parts that stick out like dogs balls in regards to being modern.

You are correct though, the restoration of your charger would suffice with a diode inserted and no caps or other components.
Though, with the switch, you should be able to get one that is very similar to the original. You could also try spraying the switch with CRC 2-26 in hopes that it will be able to slide and actually make a proper contact with the 6 volt side of it.
Regards,
Relayer

mine got a drop of deoxit in the switch.other than a sanding and repaint with the nearest rattlecan i could find its stock.the cover was rusty.
little to go wrong in these.
not bad for something older than me.

if you mean the diode's anode connects to the negative battery terminal, yeah that looks right. Should always test in DC mode, and ensure that you get a negative voltage when you reverse connections. The ripple from the halfwave rectifier will happily read weird voltages for non-true RMS voltmeters...